Patent GB-926688-A (GB)
Title: Signalling system
IPC: H04Q9/16
Filed: 1960-06-17
Abstract: 926,688. Selective signalling systems. STANDARD TELEPHONES & CABLES Ltd. June 17, 1960 [June 19, 1959], No. 21373/60. Classes 40 (1) and 118. In a remote control or supervisory system a plurality of subscriber stations are connected in series with a main station over a closed loop, n devices, one or more of which are located at each subscriber station, are selected in turn by a train of n pulses, transmitted by the main station over the loop and counted at each subscriber station, and in order to indicate abnormal states of the respective devices, the corresponding pulses of the train are modified as they pass through the subscriber stations containing the devices. The modification may be effected by altering the amplitude of the pulses or by imposing an earth or open circuit on the loop as more fully described below. Synchronizing pulses may be included in the train (a) by " stretching " certain pulses so that they perform both synchronizing and selecting functions (auxiliary sync. pulses, Fig. 2), or (b) by inserting additional pulses of different character, e.g. duration (auxiliary sync. pulses, Fig. 4). In the cases shown, each subscriber station has four devices and the lower stages of the counter thereat are reset by the auxiliary sync. pulses, i.e. after every fourth pulse of the train. A main synchronizing pulse is transmitted between successive selection trains to reset all stages of the counters. Command pulses for the respective devices may be included in the train by modifying the amplitude of the appropriate selection pulse or by inserting a pulse in the interval immediately following it, as shown in Fig. 4. The command pulses may be used to perform a test in a fire or burglar alarm system or to notify a subscriber that his premises are properly closed. Fig. 5 shows two typical subscriber stations connected in the loop. Signal separator 50 distributes the selection pulses and synchronizing pulses to the appropriate inputs of counter 51 and any command pulses to command gates 56 to energize the appropriate command output in the local terminal equipment under control of matrix 55. The matrix 55 also opens reply gates 57 at the appropriate instants so as to pass the information representing the states of the devices at that station to a keying control 64 and keyer 49. A short delay is imposed at 65 so that the first part of the selection pulses always passes unaltered through keyer 49 and on to the following stations in the loop to be counted thereat. In one embodiment the devices have three states: normal, trouble, and alarm. In the normal state the corresponding selection pulse is passed unmodified through keyer 49. Alarm and trouble states cause keyer 49 to impose an earth or open circuit respectively on the loop after the first part of the pulse has passed through. The pulse train may thus become progressively more mutilated as it passes through the loop. The district and control stations shown in Fig. 6 may be combined in a single main station. Any open circuits or earths imposed on the loop are detected by detectors 83, 84 and stored in register 85. Other subscriber loops may be connected to the central station through multiplexer 87. The central station includes a bank of lamps to indicate the states of the devices, and an audible warning device which is operated whenever an alarm or trouble condition is detected. In order to reduce the possibility of transients on the loop causing erroneous indications to be made, an alarm or trouble indication is not displayed until that state has been detected in at least two successive scans of the devices. A similar principle is employed with command pulses. Operation with a fault on the loop. The system is modified to ensure detection of a fault even though the loop is permanently earthed or broken. Each subscriber station counter is provided with an additional stage which gates an existing alarm condition so that the keyer 49 earths and breaks the loop in alternate scans of the devices. In this system only alarm and normal states are allotted to the devices. The occurrence of a permanent earth or break on the loop is detected at the district station and, by means of fault transfer and control units 82, 97, the selection train is applied to each end of the loop, and detectors 83, 84 then perform the same function for their respective sides of the loop.